Is singing one of those abilities that you either have or don't have?

I play guitar, and i'm average at it. And I've been singing for quite a while but the other day someone told me, "devon, look, you can't sing, so just shut up". So i went home and recorded myself singing and i almost threw up when i heard the result... What i want to know, is it possible to learn how to sing, even if its just average? I don't want to be amazing, i just want to be able to sing along without being told to shut up? Or am i doomed to remain a poor singer? :(